The premise sells itself. Two cultural icons meet for the first time onscreen, both reprising their roles as boxers in Oscar-winning movies. What fan wouldn't want to see Rocky Balboa fight the "Raging Bull" himself?

According to director Peter Segal, Robert De Niro and Sylvester Stallone were the only two actors he considered for the leads in Grudge Match. Executives at Warner Bros. agreed, which meant that the movie wouldn't be green-lit unless the director could cast both stars. De Niro signed on early for the part of Billy "The Kid" McDonnen. Stallone took more persuading.

"Both Bob and I convinced Sly over a period of a couple of months," Segal says by phone from his office. "He didn't want to be seen as 'winking' at Rocky. Now he's delighted. After a test screening, he told me, 'I've got a successful movie and I didn't even have to hold a machine gun.'"

Set in Pittsburgh, Grudge Match reunites Kid McDonnen and Henry "Razor" Sharp, Stallone's role, 30 years after they battled to a draw in the ring. Kid built on his success as a boxer to become a car salesman and restaurateur. Sharp, on the other hand, withdrew from life after he retired from the ring, and now works in a steel mill.

Stallone's part went through several adjustments during script development, as did the other roles. In fact, Segal added and subtracted characters throughout the process, always with the goal of making the leads more realistic and appealing.

"The original script was a little jokier," Segal recalls. "What I responded to was how preparing for the fight heals relationships, gives these guys a second chance at redeeming themselves. Sly reminded me that the original Rocky was really a love story. I think the fight itself was only seven minutes or so. This movie is a similar kind of journey."

The director and De Niro worked together to refine Kid's character. "We discussed who he was, the kind of bravado he had, he's a tortured soul, not very kind at first," Segal explains. "But Bob was also concerned about the physical aspects of the role. He asked, 'How do you want me? What kind of shape?' I said could he possibly get to Cape Fear shape, about ten pounds heavier than he was in Raging Bull. And he did, he lost 30 pounds and trained so hard that he got up to a hundred pull-ups in his workouts. So I challenge anyone who implies that someone might be cashing in on his legacy here or just taking a paycheck. You go through this physical torture. There was no easy ride here."

Segal decided to jettison Kid's love interest in the script, playing up instead his reconciliation with his estranged son B.J. (Jon Bernthal). "And originally that role was completely different," Segal notes. "B.J was a yoga instructor from Los Angeles, now he's a football coach in Pittsburgh."

Stallone also had input in adjusting his Razor character. "We had Razor kind of beaten down in the beginning," Segal says. "A little more sad sack than he ended up being. Sly's very intelligent idea was 'Let's show his pain through laughter, let him put on airs, pretend he's well-adjusted,' even though we can see that deep down he's still bitter about what happened to him."

Segal downplays his work with the stars, even though Stallone delivers one of the most natural, unaffected performances of his career. Adding a few light touches to dramatic scenes helped Stallone work out his approach. "He handled the dramatic stuff because he really understood this character," Segal explains, "and then he allowed me to kind of guide him into this really distinctive dry comic delivery." It doesn't hurt that Stallone is working in many scenes with Oscar-winning actors like Alan Arkin and Kim Basinger.

"Comedy is about being real," the director says. "Less is more. When I work with actors who say 'I can do comedy,' it's like an alarm for me because if a person thinks they have to 'do' comedy, they're already going to overdo it. That said, this was one of the most pleasant sets I've been on. Everybody was willing to try anything."

Although he encourages improvisation, Segal is careful to work from a blueprint. "Any actor who is good at improv wants that foundation," he says. "You never want to go into a scene like, 'I have no idea what we're doing, let's just be funny.' That never works. But look, Alan Arkin is one of the founding fathers of Second City in Chicago. If you don't open up a Ferrari on the track, you're an idiot."

Arkin contributed his own ideas as Stallone's retired corner man, and stand-up comedian Kevin Hart, who plays a producer who stages the fight, worked up alternate jokes with his writers. "I love that part of the process," Segal says about improv. "Especially back in the edit room. It feels like you have a lot of golf clubs in your bag. Different clubs for different reasons."

Improv can also lead to problems. Bernthal joined Grudge Match after working on Martin Scorsese's The Wolf of Wall Street. He has a key scene with De Niro over breakfast in a diner. "Scorsese's process, which of course Bob is very familiar with, is to improvise a lot, barely do the script as it is on the page," Segal notes. "But in that scene there were a couple of moments they both had to hit. Bob actually helped bring Jon back in. He said, 'Okay, you can't improvise too much here. I'm the guy on the other side of the net and I have to hit the ball back to you.' It took them several takes but they found a rhythm which I thought was wonderful."

This is Segal's fourth film with cinematographer Dean Semler, and their second together shooting digitally. Segal thinks the digital process helps actors. Steve Carell once told him he would get intimidated hearing film roll through the camera magazine, "like he would be wasting money if he got something wrong. And for directors, every time you yell 'Cut' it's like shutting off a lawnmower engine. Getting the energy, the rhythm back up is so hard, it's easier to keep the camera rolling, which you can do with digital."

Unless you roll them too long. Segal says their cameras overheated during the shooting of Grudge Match's climactic fight. "I think I yelled 'Action' on Monday and 'Cut' on Friday," he jokes.

Segal had only four days to shoot the fight, which took place in front of 500 extras at the Lakefront Arena in New Orleans. "To put it in perspective, the fight in Rocky 4 took four weeks to shoot. Scorsese shot for six weeks for Raging Bull."

Schedules were so compressed that De Niro and Stallone didn't enter the same ring until a few days before shooting. Stallone—in the director's words the best fight choreographer in the business—wrote out a detailed 31-page script of every punch and move in the bout. Segal videotaped rehearsals of the fight with Stallone in Beverly Hills, then brought the footage to New York to work with De Niro.

So both actors were more than prepared when it came to the actual fight. Segal and Semler worked with seven cameras and two operators hired from HBO Sports, as well as real-life boxing referee Pat Russell.

"Obviously we had to shoot in sequence, because you lay out a choreography of cuts and bruises," Segal says. "And you know many times fake movie punches become real movie punches. You can't do these moves without getting so close you make contact. You can see in slow motion the sweat flying when Bob's fist connects with Sly's face, and vice versa."

Segal, whose credits include the hits Tommy Boy,Anger Management, 50 First Dates and Get Smart, relies heavily on test screenings, citing the Marx Brothers and Harold Lloyd, earlier comedians who tried out their material before audiences. "We go in with a very fat cut," he says, "knowing that the audience is going to help shape it. Not just where the jokes are, but also where we're trying to get a certain feeling or emotion. Are we rushing, going too quickly? That kind of thing."

Early focus-group testing showed that viewers loved Arkin's character, so Segal decided to film a tag scene for him. And while he insists Grudge Match isn't a parody, the director did slip in a couple of direct references to Rocky. In the first, Razor has to down a half-dozen raw eggs.

"When I put that in there, I thought it could go two ways with Sly. He could really like it, or be offended. Luckily, it was the former, and he actually asked if we could do another one. 'I don't want to go crazy, but one more might be nice' was how he put it."

Segal's future projects include a version of Johnny Quest for Warner Bros. and a new take on the classic play Harvey, this one scripted by Jonathan Tropper. But in his mind it will be hard to top the experience of making Grudge Match.

"Just seeing those two actors, those two legends, in the ring together gave me chills. Bob was so dialed in, he challenged me several times. He'd ask, 'Can I try something?' that would really push the envelope but more often than not bring the house down."

As for Stallone, the two had many dinners together. "He is so smart," Segal enthuses. "He has so many stories about whenever things got tough he would write himself into another victory. He wouldn't just sit there waiting for a script to be handed to him. When he got cold, he invented The Expendables and reinvented himself. With that and of course Rocky and Rambo, who else can say they've created three movie franchises?"